Find the local maximum and minimum of $LaTeX: \displaystyle f(x) = - 5 x^{3} - 6 x^{2} + 3 x + 4$ .

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = - 15 x^{2} - 12 x + 3$ . Solving $LaTeX: \displaystyle - 15 x^{2} - 12 x + 3 = 0$ gives $LaTeX: \displaystyle x = \left[ -1, \ \frac{1}{5}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( -1 \right) = 18$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(-1\right) = 0$ is a local minimum.
$LaTeX: \displaystyle f''\left( \frac{1}{5} \right) = -18$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(\frac{1}{5}\right) = \frac{108}{25}$ is a local maximum.